Grab and save the branch number from eHerkenning service restriction

[sergei-maertens]

Closes #3967

This is really hard to test/try out because we don't have a real eherkenning setup with a branch service restriction as far as I can tell...

However, piecing together the documentation on: https://afsprakenstelsel.etoegang.nl/Startpagina/v2/interface-specifications-dv-hm (which describes the interface between service provider and makelaar), we should get back the ServiceRestriction SAML attribute if information is available in the MR (machtigingsregister). The examples show that it would not be an encrypted attribute (it sits in the AttributeStatement element):

<saml:Attribute Name=urn:etoegang:1.9:ServiceRestriction:Vestigingsnr>
    <saml:AttributeValue xsi:type=xs:string>123456789012</saml:AttributeValue>
</saml:Attribute>

The documentation says it would be one or more restriction, so we're assuming that it returns a list of strings of values after processing, similar to the urn:etoegang:core:ServiceID and urn:etoegang:core:ServiceUUID attributes.

I checked our code in django-digid-eherkenning, and we already by default include the service restriction request in the catalogus request, so no extra work should be needed there, see: https://github.com/maykinmedia/django-digid-eherkenning/blob/0189aceea660d2f4774d238397365f17adeb354a/digid_eherkenning/models/eherkenning.py#L234

Changes

• [x] Grab the service restriction from the extracted attributes
• [ ] Add tests (preferably VCR with a real setup and no mocking)

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [x] Commit messages refer to the relevant Github issue
  • [x] Commit messages explain the "why" of change, not the how